1. Field of the Invention
The invention is related to the field of Doppler measurements, and in particular, to processing a Doppler measurement signal.
2. Statement of the Problem
Instruments exist which measure fluid velocity by emitting an ultrasonic carrier signal that echoes off particulate matter carried in or on a flowing liquid and returns with its mean frequency shifted by the Doppler Effect. Other instruments use microwave energy, and it is believed that Doppler-shifted return energy is due to Bragg scattering. One common usage for such instruments is in the measurement of open-channel flow, as for instance in a wastewater collection sewer. Instruments of this type estimate fluid velocity from the observed Doppler frequency shift.
The Doppler shift can comprise a shift in a frequency of the reflected signal versus the originally transmitted signal, due to motion of an object toward or away from the Doppler transmitter device. The Doppler shift can be subsequently processed to determine the velocity. Further, the change in frequency of the reflected signal can be used to determine the direction of motion of the object, either toward or away from the Doppler measurement device.
In the measurement of fluid flow, a Doppler shift can be measured using continuous or pulsed waves transmitted through a fluid in order to detect a fluid velocity of the fluid flow. Consequently, the waves can be transmitted from within the fluid, including parallel to or at an angle from the fluid surface. Alternatively, microwave radio energy may be transmitted through the air above a fluid, impinging upon the fluid surface at an acute angle. A Doppler shift can be measured in the energy reflected from the fluid surface.
The Doppler measurements can detect and quantify a fluid velocity by measuring a movement of particles or reflectors in the fluid, such as foreign matter, air bubbles, or a movement of microwave reflectors at the fluid surface.
Various methods exist for processing of the returned signal, but most involve some type of spectral analysis. Typically, the normalized power spectral density (PSD) of the returned signal is used as a surrogate for the probability density function (PDF) that describes aggregate particle velocities. In some instruments, the magnitude spectrum of the returned signal is used instead of the power spectrum. The magnitude spectrum and the power spectrum are two types of velocity spectra. The velocity spectrum is then used to estimate mean velocity, peak velocity, maximum-likelihood velocity, or some other statistic that is relevant to flow.
In cases where the return signal is converted to digital form, it is well known that the required spectral analysis can be done using smoothed or averaged periodograms Smoothing and/or averaging is done to reduce/eliminate noise and outlier spikes in the computed spectrum. Computational efficiency is improved by use of the Fast Fourier Transform (FFT). The velocity spectrum is thus estimated at a discrete set of frequencies or bins. A back-end algorithm processes the bin values to obtain the desired flow statistic.
From the foregoing, it will be understood that it is common practice to analyze the reflected signals in the frequency domain, and frequency domain analysis is essential in estimating and processing a velocity spectrum. Frequency domain analysis at frequencies distinct from the carrier frequency will reveal a Doppler measurement response amplitude representing the Doppler shift in the reflection from the fluid. The fluid velocity is related to the amount of Doppler shift in the reflected carrier wave, wherein a large fluid velocity will result in a large shifted distance from the carrier frequency. The position of the reflected (and shifted) carrier wave with respect to the original carrier frequency is related to the velocity and direction of the fluid flow.
A two-sided velocity spectrum will often contain two obvious peaks, as is shown in FIG. 2. The central peak is associated with non-Doppler-shifted carrier wave energy. The carrier peak is central in the two-sided case. For a one-sided spectrum, the carrier peak is typically at the left side of the spectrum. The carrier wave energy is present in the return signal due to some combination of crosstalk and reflection from stationary objects, such as a flow channel boundary or other boundary surface. The carrier peak will frequently be the highest peak in the velocity spectrum and will be comparatively narrow. The other peak will represent the measurement reflection obtained from (and representative of) the fluid flow. The location of this peak will depend on the speed and direction of the fluid flow. A fluid flow in the opposite direction will be on the other side of the central peak. In either case, a faster fluid flow will always be located farther from the carrier peak.
Many instruments utilize a one-sided velocity spectrum. In these instruments, the carrier peak may be at the left-most edge of the spectrum and the flow peak will be to its right, regardless of the actual flow direction.
However, there are some difficulties in processing a Doppler measurement signal in order to generate the fluid velocity measurement. For example, noise will always be present in any measurement system. Also, the Fourier Transform itself is prone to spurious frequency spikes due to the “long tailed” statistics of the bin values. As a result, there will typically be noise artifacts in the resulting frequency spectrum. In addition, the Doppler transmitter may receive waves that are reflected from other objects, such as signals reflected from sides or boundaries of the fluid channel, from stationary objects within the flow, etc. In addition, the original transmitted carrier wave will be immediately received by the Doppler instrument and will present a very strong reflection signal in the frequency domain and substantially at the carrier wave frequency, potentially dominating the frequency spectrum. These various artifacts may make it difficult to discriminate the desired Doppler velocity measurement. All of these various artifacts need to be detected and/or removed from the signal in order to ensure that the resulting velocity measurement is accurate.
It can be observed in FIG. 2 that the estimated velocity spectrum has a non-zero noise floor. Noise may interfere with the determination of the statistic or measurement of interest. For example, it is difficult to determine the maximum flow velocity due to noise in the adjacent frequency bins. The calculation of mean velocity is corrupted by both noise and by the carrier peak. Accordingly, it is desirable to remove noise and crosstalk from the estimated spectrum prior to computing the flow statistics of interest.
In the prior art, U.S. Pat. No. 5,557,536 to Nabity et al. discloses a flow measurement system using submerged ultrasonic transducers to obtain a Doppler-shifted return signal. The return signal is digitized using complex sampling at the carrier rate, and the resultant samples are processed by means of an FFT to obtain an estimated Doppler spectrum. The statistic employed in Nabity is an “average velocity” which is apparently the centroid of the power spectrum, wherein the frequency bins are processed in order to determine a centroid of the entire result (possibly including spurious signals). This is in contrast to an earlier patent by Nabity, U.S. Pat. No. 5,371,686. The claims in the earlier Nabity patent are consistent with use of the magnitude spectrum of the reflected carrier wave.
In the prior art, U.S. Pat. No. 5,821,427 to Byrd discloses a flow measurement system using submerged ultrasonic transducers to obtain a Doppler-shifted return signal. A one-sided spectral estimate is computed by means of an FFT. The technique employed is maximum velocity and the back-end algorithm attempts to improve the estimation of this technique through least-squares curve fitting of the power spectrum.
In the prior art, U.S. Pat. No. 5,315,880 to Bailey discloses a flow measurement system which projects microwave radar energy at the top surface of an open channel. Doppler-shifted microwave energy is reflected back to a radar velocity sensor. Those skilled in the art will recognize that a velocity spectrum may be measured by Bailey and the bin values of the resulting velocity spectrum may be processed in order to derive some statistic related to flow velocity.
In the prior art, U.S. Pat. No. 5,811,688 to Marsh discloses an instrument similar to that described by Bailey. This instrument is known to use a one-sided FFT. This patent does not disclose how the Doppler estimate is obtained, although the instrument diagnostics and manual make it clear that a one-sided FFT technique is used.
In the prior art, U.S. Pat. No. 5,421,211 to Heckman discloses a squelch threshold set 40 dB below the spectral maximum value. Presumably, this is done in hopes of removing noise prior to subsequent calculations.
In the prior art, U.S. Pat. No. 5,226,328 to Petroff discloses the use of smoothing on a two-sided FFT. However, no beneficial use is made of the FFT's two-sidedness.
In U.S. Pat. No. 7,672,797 to Petroff, hereby incorporated by reference in its entirety, discloses a method of using a non-flow side of a two-sided Doppler measurement spectrum for the purpose of establishing the direction of flow and to produce a noise estimate used in a subsequent thresholding. However, this patent does not address the problem of removing substantially symmetric spectral artifacts, as provided by the present patent application. Furthermore, the patent does not teach the symmetric application of thresholding as provided by the present patent application.